In the PON system, the OLT and the ONUs perform communication in synchronization with each other to prevent data in an upstream direction transmitted from the ONUs from colliding with one another. The OLT plans to give transmission allowance to the ONUs to prevent the data in the upstream direction from colliding with one another. In the planning, the OLT takes into account delays due to distances between the OLT and the ONUs. Therefore, the OLT measures round-trip times between the OLT and the ONUs. However, because there is fluctuation in a transmission line such as jitter and wander in transmission by an optical fiber, it is necessary to periodically perform the measurement.
On the other hand, data communication is not always performed. The data communication is not performed at all, for example, at night. However, the measurement of round-trip times is periodically performed irrespective of presence or absence of the data communication as explained above. The ONUs are always set in a communicable state for the measurement of round-trip times even if the data communication is not performed. This leads to a waste of electric power. Therefore, a technology for intermittently transitioning the ONUs to a power saving state by requesting a shift to the power saving state from the ONUs is considered. A PON system that, when there is no upstream data from an ONU, does not uselessly allocate a transmission bandwidth to such an ONU and improves throughput is considered (Patent Literature 1). In this PON system, when an OLT detects a state in which there is no user data for a fixed period set in advance, the OLT deletes the registration of an ONU and notifies the ONU that an optical link is temporarily stopped. Thereafter, no transmission bandwidth is allocated to the ONU and transmission of a frame for maintaining a link is suppressed. Therefore, the ONU can reduce the number of times of transmission of the frame.